A base station of a cellular radio network requires an accurate clock signal to guarantee high frequency stability and accurate timing on the air interface. GSM specifications require a relative accuracy of 5×10−8 on the air interface which might be relaxed to 10−7 for base stations used in a pico cellular environment. This high accuracy is achieved by conveying a clock signal as a pulse train along the national telephone backbone, along the GSM infrastructure, e.g. Mobile Switching Centre (MSC) or Base Station Controller (BSC) up to the base stations.
The national reference clock has a relative frequency stability of 10−11 over 24 hours. But the long transmission chain to the base station introduces jitter and wander in the clock signal. The base station relies on an accuracy of 1.5×10−8 at its 2 MBit/s PCM (Pulse Code Modulation) Abis interface. The transcoder inside the base station has a 16 MHz clock (divided down to 2 MHz) which is phase locked to the PCM clock pulses, jitter and wander above 2 Hz is filtered out, and the signal is averaged over approximately 15 minutes. The 2 MHz clock signal which has been “cleaned” in this way has an improved accuracy and serves as a reference clock for a 26 MHz clock in the Base Station Controller Function (BCF). All frequencies and timing on the radio interface are ultimately derived from this 26 MHz clock.
The described known method of providing the base station with an accurate clock relies on a continuously existing transmission chain from the fixed network to the base station. This becomes a problem if part of this transmission chain runs across a non-clocked network, which is the case for the new indoor cellular radio networks. In these networks there usually is no BSC but the functionality of the BSC is distributed over an IP (Internet Protocol) network, or intranet. IP networks are not clocked since they operate asynchronously, and transmission times are highly variable and unpredictable.
One solution to the problem is to equip a network element with a highly accurate clock and the clock signal is distributed to a base station with a synchronous line, e.g. an ISDN (Integrated Services Digital Network) or HDSL (High Bit Rate Digital Subscriber Line) transmission line.
The goal for the indoor cellular radio networks, however, is to take advantage of the existing network cabling in office environments by connecting base stations directly to an asynchronous network.
Providing additional cables for carrying a clock signal works against the primary reason for using the intranet: making better use of an existing network. With additional cables there is no need to connect base stations to the LAN (Local Area Network) at all. Base stations can then be directly connected to the network via HDSL transmission which is synchronous and which only requires a simple twisted pair cable.
There exists a wide variety of clocks which can be used at base stations. Very expensive clocks require a constant temperature environment (oven maintained) and provide a high accuracy approaching even that of the national reference clock. To increase the cost efficiency of the system, expensive, oven maintained clocks should be avoided as far as possible, especially at base stations.